1. Field of the Invention
The present invention relates to a spectrophotometer.
2. Description of the Prior Art
The spectrophotometer is a device for determining spectrophotometrically optical characteristics of a sample substance to be analized. The optical characteristics may encompass transmittance, reflectivity, absorbance or the like optical properties. Among the spectrophotometers, the double beam type and the two-wavelength type spectrophotometers are familiar in the art. In the double beam type spectrophotometer, the light beam from the monochromator is split into a reference beam which is destined to be transmitted through a reference and a sample beam to be transmitted through a sample to be analyzed. The optical characteristics of the sample is determined by deriving ratio between the reference and the sample beam in term of the quantity of light in combination with the wavelength scanning of the monochromator. In the case of the two-wavelength spectrophotometer, the sample is irradiated with light beams from two monochromators. In carrying out the measurement or analysis, wavelengths of two monochromatic light rays may be fixed at different values, or one of the wavelengths may be fixed at a certain value while the other is scanned, or alternatively two monochromatic light rays may be maintained at a predetermined wavelength difference which in turn is scanned.
Now, difficulties of the hitherto known spectrophotometers will be described by taking an example of the double beam spectrophotometer of direct ratio recording system. Light from a light source is directed to a monochromator for deriving a monochromatic light ray which is then split through a rotating sector mirror into two light beams, i.e. the reference beam and the sample beam on the time-series base. These two types of beams are transmitted through a reference and a sample, respectively, and subsequently synthesized into a single beam by means of another sector mirror rotated in synchronism with the first mentioned sector mirror. The synthesized single beam is applied to a photodetector on the time-series base. An output signal from the photodetector is amplified and then discriminated in respect of phase with the aid of a synchronous signal generated in synchronism with the rotation of the sector mirrors. The signals corresponding to the reference beam and the sample beam (reference beam signal R and sample beam signal S) derived from the phase-discrimination are then alternatively fed to respective hold circuits thereby to be converted into corresponding d.c. signals which are then compared with each other by a ratio meter or the like to be displayed on a display such as a recorder. When the absorbance characteristic of a sample is to be determined, the measured value is often subjected to a logarithmic conversion to produce a linear display absorbance, since the absorbance is frequently discussed on the basis of the Lambert-Beer's law.
In connection with the wavelength scanning in the spectrophotometers, it is noted that the reference beam signal R and the sample beam signal S are varied as a function of the wavelength .lambda. in accordance with multiplicated values of energy E (.lambda.) of the light source, efficiency M (.lambda.) of the optical system and sensitivity D (.lambda.) of the photodetector. The rate of such variation in dependence on the wave-length is on the order of several tens to several hundreds of magnitudes in the case of spectrophotometers for ultraviolet and visible ranges. Accordingly, when the ratio (S/R) between the reference beam signal and the sample beam signal is to be determined through an electrical comparator means such as a ratio meter having usually an accuracy capable of displaying the result with three to four digits, the ratio may be determined with a reasonable accuracy in the range of the wavelengths in which the multiplicated values are large. However, in the wavelength range in which the multiplicated values are relatively poor, the attainable accuracy would be degraded to one several tenths to several hundredths as compared with the former case.
In addition to the direct ratio recording system, there has been known an automatic gain control method. In accordance with this method, no ratio meter is employed. Instead thereof, the output signal from the hold circuit corresponding to the reference beam is compared with a predetermined value through a comparator and fed back to control the sensitivity of the photodetector through a sensitivity regulator including DC-DC converters or the like, thereby to maintain the magnitude of the reference beam signal R output from the photodetector. On the other hand, the sample beam S is supplied to a display such as a recorder, whereby the ratio S/R, namely, transmittance of the sample can be obtained. This system is referred to also as the dinode feedback method. Further, method of adjusting the slit of the monochromator (slit servo method) is often employed. These prior art spectrophotometers are described in a Japanese literature, Sibata "Spectrum Measurements and Spectrophotometers" published by Kodansha Scientific, Oct. 10, 1974, pages 125-133. Furthermore, there are methods that control the intensity of the light source, a gain of an amplifier, a comb in the light beam and so on.
The spectrophotometers which are operated through analog techniques as described above are disadvantageous in that the measurement accuracy is relatively low due to drifts in the amplifiers and hold circuits as well as a poor stability of a reference potential. In particular, in the case of the dinode feedback method in which the sensitivity regulator including DC--DC converter or the like tends to respond with some delay, the wavelength scanning can not be effected as desired at a high speed, since the delay in the response of the sensitivity regulator will exert adverse influence to the spectrum as derived. In the slit servo method, change in the slit width will bring about variations in the wavelength. Further, because of regulation of a mechanical nature, delay in the response will be inevitable.
Although the foregoing discussion has been directed to the double beam spectrophotometer, it will be understood that similar problems may arise in the two-wavelength spectrophotometer since the direct ratio recording system as well as the automatic gain control method are adopted also in the latter.
In brief, the hitherto known spectrophotometers suffer from the drawback that the measurement accuracy remains at a relatively low level due to various factors, as will be appreciated from the above description.